Superplastic deformation is defined as the deformation of a workpiece to a very large strain by application of a small stress without disrupting the mechanical integrity of the workpiece. Although superplastic deformation is universally characterizable by the formula ##EQU1## (in which .epsilon. is strain rate, A is a materials constant, .sigma.is stress, R is the gas constant, T is temperature and n is a stress exponent between one and two), this behavior can be produced by any of several different mechanisms. This phenomenon has been exploited in superplastic forming techniques. For example, titanium-based materials are desirable for their specific strength and stiffness at ambient and elevated temperatures but have high resistance to deformation at temperatures appropriate for traditional hot-working operations. However, titanium alloys having a fine, stable grain structure deforms superplastically, a phenomenon known as "fine-grain superplasticity". Titanium-forming techniques based on fine-grain superplasticity only operate successfully within a restricted window of process parameter values. For example, only small strain rates can be imposed, so the process output rate is limited. The deformation mechanism requires that grain size be maintained within certain limits throughout the deformation process.
In another superplastic mechanism, called "transformation superplasticity" (described, e.g., in U.S. Pat. No. 5,413,649, the entire disclosure of which is incorporated herein by reference), the workpiece is cycled through a phase transformation by changing the temperature. The technique is advantageous compared to earlier approaches in that it is not limited to a workpiece material with a fine-grain microstructure and the grain growth limitation is relaxed. Also, the higher strain rates achievable result in more efficient process output. However, prolonged residence at high temperatures as required for some thermal cycling procedures can promote grain growth to sizes deleterious to the mechanical properties of the finished product. Implementing the required temperature cycling capability can be costly and difficult. Also, repeated thermal cycling can promote fatigue of the treatment apparatus.